Seal rings, which are rings composed of metals such as copper, aluminum, or gold, are fabricated around the perimeter of ICs to prevent IC cracking during the dicing process. An additional benefit in using a seal ring is that it serves as a barrier to moisture. That is, over time, as the IC is exposed to moisture, the IC performance may degrade without the use of a seal ring.
However, use of these seal rings also has disadvantages, including unwanted noise/signal propagation. Many times, a noise source, such as a digital signal input/output pad, clock input pad, power amplifier, etc., located on the IC causes noise to propagate along the seal ring, which subsequently can interfere with the operation of other devices on the IC. This is problematic because the devices on the IC are not then isolated from each other as may otherwise be intended and designed.
Conventionally, seal rings have taken two forms. First, single seal rings have been used that are composed of one solid piece of metal. The substrate below the seal ring is usually doped and then connected with vias to the bottom of the metal to make the best substrate connection to the metal seal ring. This helps reduce moisture contamination of the IC; however, this type of ring allows for noise signals to propagate from a noise source along the seal ring to other devices on the IC.
To alleviate this noise propagation issue, a single seal ring has been used which incorporates several “breaks,” or intermittent interruptions. This helps to reduce the noise propagation problem, but poses a risk to reliability because moisture can enter the IC through these breaks in the seal ring and degrade the device performance over time.